Technical Abstract:
A polygalacturonase-inhibiting protein (PGIP) was purified from mature 'Golden Delicious' apple fruit. The protein was cell wall bound and had a molecular mass of 44 to 54 kDa as determined by SDS-PAGE. Chemical deglycosylation of purified apple PGIP released a 34 kDa polypeptide, suggesting that differential glycosylation accounted for the heterogeneity in molecular mass. Apple PGIP showed differential inhibitory activity against five polygalacturonase isozymes purified from Botrytis cinerea grown in liquid culture. However, inhibition was not detected against polygalacturonase extracted from apple fruit inoculated with the same fungus. Kinetic studies suggested a mixed-type inhibition. The N-terminal amino acid sequence of apple PGIP shared 96 percent, 68 percent and 60 percent identity with those from pear, tomato and bean, respectively. A segment of the PGIP encoding region was cloned using the information from amino acid sequences. PGIP gene expression levels and disease reactions t Penicillium expansum and B. cinerea at different fruit maturities have been determined. The amount of PGIP transcript in apple fruit is inversely related to the size of decayed tissue caused by both pathogens, suggesting that PGIP contributes to the general defense mechanisms of apple fruit against fungal infection.